1. Field of the Invention
This invention relates to a sheet molding material of a thermosetting resin and, more particularly, to a three-layer sheet molding material, and to a process for its production.
2. Description of the Prior Art
Known conventional sheet molding materials of a thermosetting resin include "sheet molding compounds" (hereinafter, "SMC" for brevity) obtained from a mixture of a liquid unsaturated polyester resin and glass fibers, inorganic filler, etc., which is thickened with magnesium oxide, etc., and thin prepregs obtained by impregnating a substrate such as paper or glass cloth with a phenolic resin or an epoxy resin.
With SMC's, it is extremely difficult to obtain a flat plate whose thickness is quite accurate because the amount of flow of the resin is great at the time of molding. Accordingly, a special mold is required, and only one plate can be produced at a time. In addition, glass fibers are oriented to a marked extent in the direction of the flow of the resin, and, therefore, directionality occurs in the mechanical properties or dimensional accuracy. Consequently, dimensional changes such as warping (laterally) or twisting (corner to corner) tends to occur in the cured SMC sheet. Furthermore, when a different kind of thin film, such as a decorative sheet, a film and a metal foil, is to be laminated on such a sheet, distortions and creases tend to occur in the thin film because the surface of the sheet is not smooth and is tacky. It is difficult, therefore, to laminate such a thin film during the molding of the sheet molding material. When the thin film is laminated after the sheet has been cured, adhesion of the thin film to the sheet is reduced.
In a prepreg, the resin is generally partially cured, i.e., to the B-stage, and the surface is non-tacky. With such prepregs, several hundred sheets can be produced at a time by using a multi-stage press which does not require a mold or a spacer. A copper foil or the like may be laminated, simultaneously, on the sheet and the resulting laminate is utilized as a printed circuit base board. Formation of these sheets, however, requires a lamination of five or more prepregs, and this operation is quite laborsome and has a poor efficiency. Furthermore, since several thin prepregs are laminated, deaeration in spaces between the layers is difficult, and unacceptable products with voids or poor adhesion between the layers will be formed. Production of prepregs usually involves impregnating a base material, such as paper or cloth, with a solution of a resin composition in a solvent, usually called a varnish, and evaporating the solvent. Therefore, pollution prevention measures must be taken to recover the solvent and prevent its dissipation outside the system. Since the viscosity of the varnish must be low in order to achieve good impregnation and to eliminate bubbles, a large amount of inorganic filler cannot be used, the cost of production cannot be reduced by conserving the amount of expensive resin used, and the latitude in improving the properties is narrow. Furthermore, it is very difficult to control the flowability of the resin in the prepreg when the resin is in the B-stage. When a large-size flat plate, e.g., with one side measuring 1 meter, is produced, there is a great difference in thickness between the central and edge portions of the plate. Hence, the area of the plate which must be trimmed off is large, and the yield of the product is low.
The present invention provides a sheet molding material of a thermosetting resin useful for obtaining a flat plate, which can be used to remove these disadvantages and on which different kinds of thin films can be laminated at the same time.
Some of the coinventors of the present invention found that a sheet molding material of an unsaturated polyester resin having an uncured surface layer and an interlayer having an average degree of curing of at least 50% can be press-formed without using a mold or a spacer, e.g., as disclosed in Japanese Patent Application (OPI) No. 101276/77. The present invention is an improvement over the invention disclosed in Japanese Patent Application (OPI) No. 101276/77.